This invention relates generally to a valved damper and more particularly, to a damper that is pneumatically controlled to provide variable damping forces.
Conventional vehicle suspension dampers typically consist of direct double-acting telescopic hydraulic passive dampers. They are generally described as either shock absorbers or struts. A primary purpose of shock absorbers is to dampen oscillations of the vehicle suspension spring. This is accomplished by converting kinetic energy in the form of motion between sprung and unsprung masses of a vehicle into heat and then dissipating the heat. Struts also serve this capacity and in addition, support reaction and side-load forces on the suspension.
Typical dampers are hydraulic devices using oil as the fluid medium for converting motion into heat. As the damper is cycled, a piston is forced to move in extension and compression directions through the oil that is contained within a cylinder tube. This creates pressure within a portion of the cylinder tube and a consequent pressure drop across the piston. During an extension stroke, high pressure is created in the extension chamber of the cylinder tube above the piston, forcing oil to flow through the valving of the piston. During a compression stroke, high pressure is created in the compression chamber of the cylinder tube below the piston, forcing oil to flow back through the piston's valving.
As oil is forced to flow through the orifices in the piston a pressure drop is effected and the oil is heated. In this way, the damper dissipates energy stored by the vehicle's suspension springs. The extent to which the oil is heated and consequently, the amount of energy dissipated is controlled by the size of the orifices in the piston and the amount of flow forced through the piston valving.
Damping force is a common measurement of the performance of a damper. It is used to quantify the amount of spring control provided by a damper. Passive dampers are tuned to provide selected vehicle performance characteristics. Because passive dampers provide a set damping characteristic they are generally somewhat of a compromise in providing optimum damping performance over a wide range of operating conditions.
The concept of dampers with an electrically controlled damping force has been developed in the art wherein an algorithm is used to provide a control mechanism as a means of varying the damping force provided by a damper. One example is shown in U.S. Pat. No. 5,690,195 hereby incorporated by reference. Electrical control is typically achieved by varying the valving orifices in response to various sensors which are used to detect vehicle operating conditions. Such dampers adjust the damping force in response to the control mechanism so that various performance characteristics can be provided by an individual damper. An electrically controllable orifice however, provides less than ideal levels of control at low flow rates.
Electrically controlled hydraulic dampers for vehicle suspensions have, in principle, been known in the art for some time. However, their actual widespread application and use have been tempered because of the expense of such a system.
One solution to these and other problems with prior art systems is disclosed in U.S. patent application Ser. No. 09/781,868, assigned to the assignee of this invention. The damper in accordance with a preferred embodiment of that invention includes a cylinder slidably carrying a piston which separates the cylinder into extension and compression chambers. The piston carries a control valve for controlling fluid flow through the piston. The control valve provides a variable amount of damping force by regulating damper fluid flow between the extension chamber and the compression chamber of the damper during extension and compression strokes. Pressure regulation across the piston is controlled through a primary and a secondary flow path in which the flow through the secondary flow path or branch is determined by the control valve. The primary flow path or branch remains open.
The flow passage through the piston includes a secondary flow path or second branch that communicates through ports in the control valve, and flow therethrough is alternately interrupted, completely or partially, as determined by the control mechanism or valve. When flow through the second branch is interrupted by the control valve, the flow passage through the piston extends only through the first branch which includes the first passive valve individually.
That invention includes a hydraulic damper that uses pneumatic control of the control valve to vary damping levels. Air pressure from an air-suspension or air-leveling system of the vehicle is vented in a presently preferred embodiment of this invention to a bellows or air-pressure actuated control valve located in the piston. The bellows or control valve compresses in response to higher pressure from the air-suspension or air-leveling system. The compression of the bellows control valve controls the position of a spool valve and as the spool valve is closed, one of two parallel flow paths through the piston is closed. Each flow path has its own valving. When only one of the flow paths is open, a higher damping force is generated. As a result, a continuously variable damper provides varied damping force according to the vehicle payload and operating conditions.
The pneumatic control of the damper uses the air pressure of the vehicle's air-suspension or air-leveling systems to control the position of the spool valve. The air pressure input to the control valve varies according to the overall vehicle weight as well as the road conditions. As the damper is stroked due to road input, the pressure varies. Higher frequency pressure oscillations resulting from road input are filtered with an orifice mounted inside a hollow piston rod in an attempt to limit damper variation to only static vehicle weight. The resulting air pressure is used as an input to the bellows or air-actuated control valve. A higher vehicle payload will result in higher pressure surrounding the bellows resulting in contraction of the bellows. As the bellows contracts or compresses, the secondary flow path is blocked by the spool valve. A higher pressure surrounding the bellows will result in the bottom face of the bellows lifting and the spool valve and thereby shutting off the secondary flow path. The pneumatic system is isolated from the piston hydraulic system with seals that are held in place with a seal plate, seal retainer and piston adapter.
While the piston and rod assembly of the invention disclosed in U.S. patent application Ser. No. 09/781,868 is a significant advance, such systems should preferably be easy to manufacture and assemble while still providing a robust and reliable damper for use in commercial production of vehicles. Factors such as ease of manufacturing and assembly as well as reliable and maintenance free construction design often are the difference between a successful or an unsuccessful commercial implementation of a unique invention such as the damper disclosed in the above-identified patent application.